In the U.S. Pat. No. 4,052,648 (Nola), entitled "Power Factor Control System for A.C. Induction Motors", there is disclosed a power reduction system for induction motors in which the operating power factor of a motor is controlled as a function of the difference between a commanded power factor signal and the operating power factor, through control of a thyristor connected to the motor.
As will appear, the present invention is applicable to power factor controllers (PFC) of this general type, or to other systems of this type, which require the measurement of the phase angle of current flow with respect to the line voltage in a threephase, three-wire system. Other schemes have been disclosed which use either current or voltage transformers to measure the current, or use optical couplers to sense the thyristor voltage while providing the required isolation. These approaches are both expensive and are sensitive to the amplitude of the current. This latter disadvantage does not necessarily hinder the phase measurement itself, but does raise the cost of manufacture by making it necessary to stock components of various sizes. A third approach achieves isolation by using a separate power supply for each phase. Since the phase information from each phase has to be summed in a common amplifier in the PFC, optical couplers (or equivanet isolators) are still required. Again, this is obviously an expensive technique. In addition, all of the above approaches require two conventional zero crossing detectors (typically formed by operational amplifiers and associated resistors) per phase to obtain a square wave and the inverse of the square wave as synchronized with the current flow.
In U.S. Patent Application Ser. No. 199,675 (Nola), a single phase PFC is disclosed wherein the phase angle of the current is measured by sensing the voltage across the thyristor. This same technique is applicable to a three-phase PFC as well. However, in the single phase, one side of the thyristor is connected to the same ground reference as the low level circuitry power supply. By simply connecting one input of the operational amplifier to ground and the other input to the high side of the thyristor through appropriate resistors, the voltage across the thristor can be sensed. However, this technique as applied to three-phase sensing requires that a high common mode voltage be rejected or ignored by the operational amplifiers. This problem, and the solution provided by the present invention, are discussed in more detail hereinbelow.